1. Field of the Invention
The present invention relates to a sintered body of low temperature cofired ceramic obtained by firing a non-glass low temperature cofired ceramic material, and to a multilayer ceramic substrate including the sintered body of low temperature cofired ceramic.
2. Description of the Related Art
A sintered body of low temperature cofired ceramic (LTCC: Low Temperature Cofired Ceramic) is obtained by forming a low temperature cofired ceramic material into a predetermined shape and subjecting this material having the predetermined shape to sintering.
The low temperature cofired ceramic material can be subjected to cofiring with a low melting point metal material such as silver or copper with a relatively small specific resistance, thus, can form multilayer ceramic substrates which are excellent in terms of high frequency characteristics, and has been used as, for example, a substrate material for high frequency modules in information-communication terminals.
While a ceramic material such as Al2O3 mixed with a B2O3—SiO2 based glass material, a so-called glass-ceramic composite system is typically used as the low temperature cofired ceramic material, this system requires the use of a relatively expensive glass material as a starting raw material, and in addition, contains boron which is likely to volatilize during firing. Therefore, substrates obtained are likely to fluctuate in composition, and thus, the manufacturing process of the substrates is complicated, such as having to use a special setter for controlling the volatilization amount of boron.
Accordingly, low temperature cofired ceramic materials have been proposed as described in, for example, Japanese Patent Application Laid-Open No. 2002-173362, Japanese Patent Application Laid-Open No. 2008-044829, and Japanese Patent Application Laid-Open No. 2008-053525. The low temperature cofired ceramic materials described in these documents will not encounter the problem as described, because the starting raw materials contain no glass, and moreover, because the low temperature cofired ceramic materials are non-glass low temperature cofired ceramic materials containing no boron.
However, sintered bodies of low temperature cofired ceramic obtained by sintering the low temperature cofired ceramic materials described in these documents may have an insufficient joint strength with a conductor film formed on the surface thereof in some case, and in addition, the sintered bodies themselves have a small fracture toughness value, and thus, may fail to provide desired strength properties in some cases.